sands of Woolwich, the clays of London and Paris, and similar formations, up to the Alluvial deposits in which we discover the remains of still existing plants and animals.

It must not be supposed that there is any artificial arrangement in this. One after the other-one on the other-these deposits have taken place; and during the vast periods which were required to effect the subsidence of sand or mud, many thousand feet in thickness, earthquake strugglings and volcanic fires have shaken, rended, and in some cases overturned, the Earth's crust, giving rise to mountain and valley, lifting to the tops of the hills beds of shells which were deposited in the depths of the ancient seas. Wonderful and soul-engrosing as those mutations are, we must not allow them to lead us from our subject, COAL. It was, however, necessary that the true position of coal amidst the rocks should be stated clearly, to enable our youngest reader to follow us in describing the probable process of coal formation. Let us remember that coal, as a rule, is found above the Old Red Sandstone rocks, and below those which possess, in many respects, a similar mineralogical character, and which are known as the New Red Sandstone. With the few exceptions to this rule, all of which are very readily explainable, it is not possible to deal in a popular article.

The Old Red Sandstone is characterized by the remains of strange fishes. There are, especially, the genus Cephalaspis, or "buckler-headed,"-so named from the extraordinary shield which covers the head: the Asterolepis, or "star-scaled," half fish, half reptile-the story of which is well told in Hugh Miller's Footprints of the Creator:-the Pterichthys, or "winged fish" and a gigantic crustacean, Pterygotus, a lobster-like creature upwards of four feet in length. Besides the fish remains which have been discovered, this old rock preserves numerous Mollusca, and a long series of Zoophytes,-sponges and corals,-uniting, as they strangely do, the vege table and the animal kingdoms. We have evidence which conclusively shows that the rocks, grouped under this general term, were formed in the bed of an ancient sea.

In the South of England, and in Wales, we find the Mountain Limestone resting on the Old Red Sandstone, and below the Coal, destitute of land plants, laden with marine creatures, and in a great measure actually formed of corals and crinoids, or "lily-like" animals. In the North of England, and in Scotland, these limestone rocks alternate with the Coal Measures, but they still maintain a marine character. More recent yet than the Mountain Limestone-though by no means so important or so extensively developed-we find a rock composed of sand, of pebbles, and angular fragments, cemented together, known as the "Millstone Grit," being, from its hardness, frequently used in the manufacture of millstones. This rock is another evidence-from its physical character and its chemical constitution of a mechanical watery origin. We next arrive at the Carboniferous series of rocks, or those which contain the Coal Measures proper. These, in many cases, are buried, with ten thousand feet of sand and mud

above them, forming the New Red Sandstone rocks, and an extensive series of more recent formations which we need not particularize here.

Such being the geological position of our Coal, we have next to consider its mode of formation; but before we can enter on the question of the origin of coal, it is necessary to state how we determine it to be of vegetable origin.

It has been said, by some microscopic observers, that a true ligneous structure can be detected in coal; this is, however, denied by our most eminent botanists. Plants, in great abundance, are found preserved in the Coal Measures; but these are not in the state of coal. However, the chemical constitution of coal clearly indicates it to have originated from plants. The vegetable world consists, essentially, of carbon, combined with the two gaseous bodies which form water-hydrogen and oxygen; and coal is formed of the same elements, differing only in the proportions in which they are combined. The progress of the change, from a living tree growing in the sunshine, to a dead lump of coal lying deep in the Earth, is indicated to us, if not exactly determined.

Every one must have observed decaying wood. Whether the decay goes on by the process of dry or wet rot, it is still a case of slow combustion. The carbon is attacked by the oxygen-that, in every way wonderful gas, which is at once the supporter of life and light, and the destroyer of all things. By this combination a gaseous acid-carbonic acid -is formed and expelled, leaving, relatively, an increased quantity of carbon behind. Thus we have dark and dusty rotten wood in the works of Art, and we have brown coal or lignite, in which the woody structure is preserved, in the products of Nature. Chemistry shows us the kind of change which takes place; and although it does not explain to us the conditions under which the change occurs, it gives us an intelligible result :

With this chemical evidence in support of the hypothesis that coal is altered vegetable matter, let us proceed to the examination of the physical conditions under which it was formed.

Geological research indicates a period in the history of our own land when the sea washed around an extensive group of low islands, formed of the older granitic and slaty rocks, from the waste of which the Old Red Sandstone rocks were forming. In the course of ages these, almost landlocked seas, became shallower, and the deposited matter rose around the margins, towards the surface of the waters. Myriads of strange and beautiful fish sported in the waves, which glowed with the reflection of a sun, tropical in the intensity of its light and heat. On the slopes of the shores the coral animals were working in those early days, as they are now

labouring in the Pacific Ocean, forming their calcareous cells, so beautifully preserved in the limestones of Derbyshire and Devonshire.

Thus, by the wearing down of the land, and by the active agencies at work in the waters, vast tracts of low, swampy lands were formed.

These vast morasses, and the shallow waters of widely-spread lagoons, became the abodes of a wild, a strange, vegetation. Tree ferns rose high into the air, and spread their fronds so thickly, that deep shadow reigned for ever in the groves. Hosts of smaller ferns, almost infinite in variety, luxuriated in those shades-succulent plants, like the Sigillaria and club mosses, abounded; and other mosses and fungi covered the damp ground. Vegetable life was abundant-to a degree which we can scarcely realize. Amongst these teeming organisms, one of the most remarkable is the plants known as Sigillaria. "They are generally," says Dr. Hooker, “but a few feet high, though sometimes two yards broad at their expanded bases; they are truncated at the top. So common are they, that I have, in many South Wales and other collieries, counted five or six in the space of a few fathoms, always suggesting the idea of the erect stumps of trees in a forest." Stigmaria-long, serpent-like roots, shooting off from a centre into the mud in which they grew-were once thought to be a peculiar, a distinct, form of vegetable growth. They are now ascer tained to be the roots of the Sigillaria. The accompanying figure will convey some idea of this remarkable plant.

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These plants appear to have been of a very lax fibre. They grew, all probability, to an enormous size with great rapidity, and as speedily decayed-forming and adding to the mass of humus which fed the mighty grasses, ferns, and mosses, clustered and matted around the larger trees.

A contemplative student of geological phenomena has created anew for us-in the drawing we copy-the general aspect of the Flora which spread over those islands when our coals were in process of formation.

Vast swampy plains like those were thickly spread with the Sigillaria, sending their vast roots far over the mud, to absorb speedily the water required for their rapid growth. Graceful Lepidodendrons grew in abundance, these were gigantic arborescent club mosses, bearing, at the ends of their graceful leaves, their cone-like fruit.

There, again, were vast spaces covered with huge "horsetails "-the

Esquisetum; and on the soft, marshy silt of the river's edge and the estuary, grew forests of reeds-Calamites. Ferns and mosses combined with these, and formed a mat of vegetation which rapidly filled the shallow waters.

Floating in the deeper parts, were found the Asterophylites. There were numerous varieties of this plant, all of them remarkable for their graceful forms, and apparently all growing in waters sufficiently deep to float their

branches and leaves. One curious form of the Asterophylites plant is represented in the annexed figure, as it is preserved, a record of the past, in the shale in which it was imbedded.

Under the influence of strong solar excitation, the vital powers of vegetable nature were stimulated to the highest. This was also quickened by a high terrestrial temperature. We have evidences proving to us "that, in the ancient world," to quote the words of Humboldt, "exhalations of heat, issuing forth through the many openings of the deeply-fissured crust of the globe, may have favoured, perhaps for centuries, the growth of palms and tree ferns, and the existence of animals requiring a high temperature, over entire countries where now a very different climate prevails." There is little doubt that such were the conditions when a teeming vegetable world drew its carbon from the atmosphere, in which it existed in the form of carbonic acid. That, under those conditions-life being at its maximum of power-these plants decomposed this carbonic acid; and, giving back the oxygen to the air, built up with rapidity their woody structures with the carbon thus obtained. These plants died, and decomposed-through the same agencies-as rapidly as they grew, forming dense beds of black matter, which were slowly resolved into the state of coal. Contemplate for a moment the length of time required to form a bed of coal, such as that which exists in South Staffordshire, having a mean thickness of thirty feet. This is unusual; but even to form a coal-bed of but one yard in thickness must have required a long lapse of ages!

It has been somewhat too hastily said, that coal is formed directly from wood, and that much of it is found to retain its woody structure. There is, as before stated, great doubt on this point. That wood may be eventually converted into coal is admitted--but in changing, it entirely loses the form of wood-retains no evidence of fibre. It may, under the influences of heat and moisture, be converted into a bituminous mass, which is eventually consolidated into coal; but we cannot discover any evidence of wood being transmuted directly to coal. The remains of woody trees found fossil in the Coal Measure strata may be silicified-may become limestone, may be iron ore certain it is, they are never coal. The probability is, that the coal mass itself was produced from cactus-like plants, from club mosses, or peat mosses, or from aquatic plants, either marine or fresh-water.

The vegetable mass, whatever may have been its origin, from which our beds of fossil fuel is derived, may have been formed from plants which grew on the spot where we now find it; and the under-clay, as it is called, is supposed to be the soil in which the plants grew; or it may have been removed by the waters in a plastic state, floated out into the deltas or seas, and eventually, in obedience to the law of gravity, have sunk to the bed of the then existing waters.

Knowing that many of these coal-beds are now several thousand feet below the surface, we have either to suppose-if we adopt the first hypothesis a gradual subsidence of the Earth to the depth at which the coal is now found; or, if we prefer the second, to imagine the filling up of the seas, after the coal has been deposited, by enormous beds of sandstone or of shale. Sir Henry de la Beche describes a section near Swansea having a